1. Field of the Invention
The present invention relates to an optical device, such as an optical device, a light receiving device for use in an optical pickup system, a hologram unit, or the like, and to a production method thereof.
2. Description of the Prior Art
In recent years, an optical device incorporated in video cameras, digital cameras, digital still cameras, and the like, has been provided as a package element wherein a light receiving region is covered with a translucent plate while an imaging device, such as a CCD, or the like, is mounted on an adapter, such as a base made of an insulating material, or the like.
For the purpose of decreasing the size of the optical device, the imaging device mounted on the adapter, such as a base, is in the form of a bear chip (see, for example, Japanese Unexamined Patent Publication No. 2000-58805).
FIG. 6 is a cross-sectional view of a conventional optical device. The optical device is mainly made of a ceramic material or plastic resin. Referring to FIG. 6, the optical device includes a base 131 which is in the form of a frame having an opening 132 in the central portion, an imaging element 135 attached to the lower surface of the base 131 which includes a CCD, etc., a translucent plate 136 made of glass attached onto the upper surface of the base 131 to face the imaging element 135 through the opening 132 provided therebetween.
On the lower surface of the base 131, a reentrant cavity 133 is formed in a region around the perimeter of the opening 132. A wiring 134 formed by a gold plated layer is provided to cover a region extending from a vicinity of the opening 132 at the lower surface of the base 131 to the outer perimeter side surface of the base 131 as shown in FIG. 6. An imaging element 135 is attached to the perimeter of the reentrant cavity 133 at the lower surface of the base 131 such that a light receiving region 135a is exposed to the opening 132.
Near the perimeter region of the upper surface of the imaging element 135, an electrode pad (not shown) is provided for transmitting a signal between the imaging element 135 and external devices. An internal terminal section is formed at an end of the wiring 134 adjacent to the opening 132. The internal terminal section of the wiring 134 and the electrode pad are electrically connected to each other with a bump (protruded electrode) 138 interposed therebetween. The imaging element 135, the wiring 134 and the bump 138 are sealed with a sealing resin 137 provided around the imaging element 135 at the lower surface of the base 131.
As described above, the light receiving region 135a of the imaging element 135 is provided in a closed space formed at the opening 132. This optical device is mounted on a circuit board while the translucent plate 136 is at the upper side of the device as shown in FIG. 6. An external terminal section is formed on the lower surface of the base 131 in a region of the wiring 134 which exists outside the area of the reentrant cavity 133. The external terminal section is used for connecting to an electrode on the circuit board.
Although not shown, a mirror tube incorporating an imaging optical system is provided above the translucent plate 136. As for the positional relationship between the mirror tube and the light receiving region 135a, the required accuracy is determined such that the relationship is within a predetermined tolerance.
Light which comes from an object to be imaged is focused on the light receiving region 135a of the imaging element 135 by the imaging optical system incorporated in the mirror tube. The focused light is photoelectrically converted by the imaging element 135.
An example of an optical device which has a structure different from that of the conventional base 131 shown in FIG. 6, e.g., a base having a generally flat shape wherein a reentrant cavity is not formed in the surface on which an imaging element is mounted, has been known (see, for example, Japanese Unexamined Patent Publication No. 2002-43554). In such a case, an external terminal section in an outer perimeter portion which is outside the perimeter of an opening of the base and an electrode on a circuit board are connected by a solder ball having a large diameter, or the like. The gap between the lower surface of the imaging element and the upper surface of the circuit board is adjusted by the solder ball.
A solid state imaging device having such a structure has a small package height, and the area occupied by such a solid state imaging device is small. Thus, this solid state imaging device is suitable for high density packaging.
The above-described structure is basically employed also in a light receiving device used in an optical pickup system which performs writing, reading and rewriting of information in/from a recording medium, such as DVDs, CDs, MDs, etc., and an optical device, such as a hologram unit in which a plurality of components of an optical pickup are integrated.